This invention relates generally to the field of plastic molding. A mold and process for manufacturing are provided for producing an article having a molded wall that is very thin. In connection with the invention, a “very thin” wall can be construed either to have a material thickness of 0.4 mm or less; or to have a ratio of molding material flow path length to thickness of 150 or more.
The relative thickness versus elongation of an article to be injection molded using heated plastic resins or the like, can cause molding challenges and design constraints. Tradeoffs that are made in molding very thin wall articles can affect the strength of the molded parts, their surface quality, cost, the speed at which they can be produced, the energy requirements for temperature control and other factors.
Some of the problems and choices that are encountered concern arrangements by which heat energy is transferred from the heated molding material into the mold cavity. The molding material is introduced into the molding cavity at high temperature and low viscosity, by force of injection pressure, so as to flow and fill the mold cavity. After the mold has been filled, cooling of the molding material to a temperature below a solidification temperature causes the molding material to set in the shape of the molding cavity. The cooling occurs by heat transfer from the molding material to the molding cavity. It is undesirable for this process to take any longer than absolutely necessary. Therefore, the molding material temperature at the time of injection and the temperature and thermal inertia of the molding cavity are determined, often by trial and error, so that the molded part is cooled and sufficiently solid to be removed from the mold, relatively promptly after the mold has been completely filled.
If the molded part is thin, the flow path of the injected material is relatively long and narrow. If the mold cavity then is kept at a temperature below the solidification temperature of the material, there is a tendency for the molding material flowing along the thin flow path to solidify, between the point of injection and the most distant part of the mold cavity. The cavity does not fill before setting material blocks the flow path. Various material and design changes might help with this situation, but have drawbacks. For example, if the mold cavity is kept at a higher temperature during filling, it takes a long time for the molded part to set. Temperature cycling arrangements require controls. If the material composition is arranged to have a particularly low viscosity, the surface or strength qualities of the part may be less than adequate. A very high injection pressure can be used to fill the mold very quickly, but that causes other design and operational problems. Some of the tradeoffs that are of concern are discussed in the publication “Elimination of Process Constraints in Plastics Injection Molding,” Abbott, et al., Dept. of Plastics Engineering, U. Mass. Lowell.
Due to these issues, thin wall parts are molded at high melt temperature, very high injection pressure, and high injection rates. For example, typical conditions for injection molding a DVD disc, which is 0.6 mm thick, include: 380° C. melt, 17,000 to 20,000 psi maximum injection pressure, and 0.11 S. fill time. U.S. Pat. No. 6,325,950—Hosokawa, et al. teaches an injection rate of at least 65 cubic centimeters per second for DVD molding.
DVD and other optical data disks can have a considerable ratio of molding material flow path length to thickness, for example approximately a factor of 100. An injection “coining” process can be used for optical disc molding by commencing molding with a low starting clamp force, such as only one or two tons. Under appropriate conditions, a low clamp force may allow the injection pressure to force the cavity surfaces further apart temporarily increasing the thickness of the melt flow path by 15 to 30 percent. After melt injection is approximately complete, the clamp force is increased. The added clamp force squeezes the melt to fill out the cavity and returns the cavity thickness to the desired final disc thickness. One brand of machine for DVD molding provides maximum injection pressure options of 25,000 or 30,000 psi with 30 metric tons of clamp capacity. A different machine for other types of thin wall parts provides injection pressure of 47,000 psi.
If the thickness of the optical disc is to be reduced, for example to 0.25 mm and the volumetric fill rate is reduced by a comparable factor relative to a nominal rate to fill a DVD cavity of 0.6 mm in the same time period, the required injection pressure needs to be increased by a factor of 5.76, approximately. At that factor, the injection pressure could need to increase to 100,000 to 115,000 psi, which is beyond the capacity of typical optical disc and thin wall molding machines.
U.S. Pat. No. 6,440,516—Yamasaki, et al., for optical discs, states that injection molding cannot produce a disc substrate less than 0.3 mm thick. It further asserts if pits and grooves are to be molded into the surface of a disc; the disc cannot be less than 0.5 mm thick. In fact it has not been practical or possible to date to dependably mold the approximately 0.3 mm thick middle layer disc to the required quality needed for DVD-14 and DVD-18 described by U.S. Pat. No. 6,177,168—Stevens, despite the substantial economic benefit that might be derived. The current process, which is expensive, molds an 0.6 millimeter thick transfer disc; applies a reflective coating to the transfer disc; bonds it to a semi-reflective coated polycarbonate disc; and then strips the transfer disc away from its reflective coating and discards the transfer disc. This leaves a semi-reflective and reflective coating separated by a controlled thickness of adhesive on the polycarbonate disc.
The ratio of flow path length to thickness for a 0.25 mm thick disc is approximately 240. This thickness is a very thin wall article in accordance with the present subject matter. It would be advantageous to develop a practical and dependable way to produce high quality molded discs at this extreme thinness.
When injecting hot plastic melt, cooler mold cavity surfaces rapidly carry away heat energy, causing the plastic to solidify at the spaced walls of the cavity while the melt flows into the cavity between the solidified plastic at the cavity walls. In other words, the melt develops frozen surface layers against opposite walls bounding the flow path, reducing the thickness of the flow channel for the melt to a thickness that is less than the space between the opposite walls.
The rate of cooling is proportional to the cavity surface area (and other parameters such as the temperature difference). The amount of heat energy in the melt is proportional to the cavity volume. A very thin wall article has a high ratio of surface area per unit of volume. Therefore, cooling of the melt can be rapid in very thin wall articles. As the flow channel for the melt is reduced to a fraction of the wall thickness by solidified melt material on the cavity walls, the melt flow path becomes thinner and the blockage becomes more prominent. For the 0.6 mm thick DVD optical disc, if a frozen layer on each opposite cavity wall builds to 0.09 mm, the injection flow path thickness is decreased 30 percent to 0.42 millimeters. The flow path thickness for the 0.25 thick disc example described above would also be reduced 0.09 mm a side, a reduction of 72 percent to 0.07 mm. If the phenomenon is addressed by increasing the injection pressure, the required injection pressure to fill the mold would be impractical or unattainable using existing equipment.
U.S. Pat. No. 6,440,516—Yamasaki et al. and U.S. Pat. No. 6,512,735—Takeda et al. conclude that a 0.1 millimeter optical surface layer disc, for information capacities over 15 GB, cannot be molded. Because of this limitation, Yamasaki '516 describes using a pressure sensitive adhesive film or a dry photopolymer film for the surface layer. Such films do not pass climate tests, are not cost effective due to material costs, and may require trimming the film after attachment, which adds a step and a work station to the process. Yamasaki '516 describes a spin coated ultraviolet cured layer. Spin coating a layer with satisfactory properties and geometric tolerance has proven extremely difficult and is not currently done.
U.S. Pat. Nos. 6,276,656 and 6,019,930, both to the present co-inventor, F. Baresich, teach techniques to optimize molding processes by reducing the total molding cycle time. These patents are not concerned with maintaining a material flow path for a very thin molded part, instead concerning techniques to minimize molding cycle time without regard to cavity dimensions. Remarkably, the cycle time is decreased by retarding the rate at which heat energy passes from the molten material into the thermal mass defining the mold cavity, using temperature boosting insulators, which one would not expect to shorten overall molding cycle time. However, the patents demonstrate that in connection with other factors including the ability to begin with the cavity at a much lower pre-injection temperature than would otherwise be necessary without the temperature boosters, the cycle time can be shortened.
The present invention is concerned with the ability to make very thin wall parts. It should be apparent from the foregoing discussion that a very thin injection molded data disc would be advantageous. It is also plain that the present technologies for making very thin parts are struggling with problems of flow restriction and pressure. Insofar as the thermal mass of the cavity is employed to receive and remove heat energy from the melt, and the melt needs to flow past cavity surfaces to fill the entire cavity, there is an inherent problem that prevents unlimited decrease in the thickness of molded parts.
Thin wall parts such as syringes have to be filled quickly, but are limited in wall thickness due to the injection pressure required to fill the long core causing some deflection of the core pins and subsequent variations in wall thickness. Running higher mold temperatures can reduce this affect but at a tradeoff to cycle time.
Jewel case for example, once they were being made in China, many manufacturers began to reduce wall thickness to save weight and cost. However, the increased need for injection speed to fill the cavity also caused stress in the hinges that caused many to break upon opening. A thinner wall section without flow induced stress and without added cycle time would be desirable. Jewel cases are made from polystyrene and the mold temperatures are generally set for below 20C.